The proposed research is intended to investigate the mechanism of the metabolic stereoisomeric inversion of chiral pharmaceuticals in the body. The bioactivation of the inactive enantiomer in the racemic drugs via stereoisomeric inversion may result in severely adverse consequences due to the enhanced toxicity, especially for the elderly and patients with metabolic disorders. This phenomenon of chiral inversion has been commonly observed in the 2-arylpropionic acids which are important therapeutic agents for musculoskeletal disorders, but the mechanism as well as the involved enzyme system remains unclear. The objectives of this proposed investigation are to verify the inversion mechanism, to identify the enzyme system, and to carry out the pharmacological evaluation of individual 2- arylpropionic acids at the molecular level. Furthermore, this research will be extended to explore the metabolic inversion of other important chiral pharmaceutical such as beta-adrenergic blockers. In the elucidation of inversion mechanism, chiral tritium-labeled 2- arylpropionic acids will be synthesized and exposed to the homogenate of the liver and other organs of different animals. From the cofactor requirements and tritium release experiments, the mechanism can be unambiguously verified. Rat, rabbit, and guinea-pig will be used to compare the inter-species differences in this biotransformation. The isomerization enzyme system(s) will be purified using the combined techniques of conventional and affinity chromatographies. Various 2- arylpropionic acids, especially those which exhibit significant adverse effects will be examined using the purified enzyme system. To further extrapolate our experimental data to humans, cultured cells from different human organs will be used to evaluate the significance of this metabolic inversion in the human body. These proposed experiments will provide us important kinetic information, so that safer drugs such as alpha-deutero- or alpha-fluoro-derivative of 2-arylpropionic acids can be rationally designed. Similar approaches will be take to scrutinize the metabolic inversion of beta-adrenergic blockers.